To an opening for discharging or introducing contents of a sealed bag that can be used for various purposes such as a sealed bag to be used for storing a fluid such as a liquid or gas or for storing futons and clothes, etc., by removing air from the inside, or used as a cushioning material that utilizes a cushioning effect of gas sealed within a small partitioned storing portion, a check valve is attached in order to accomplish the purposes (Patent Documents 1 and 2).
The check valve limits the direction of the flow of a fluid such as a gas or liquid inside and outside the storing portion of the bag to only one direction.
There is a check valve, in particular, to be used for preventing air from switching with a liquid and flowing into a sealed bag when the liquid is discharged from the sealed bag, etc., storing the liquid (Patent Document 2).
In detail, the check valve includes two outer sheets and a valve sheet each made of synthetic resin, and the outer sheets are layered with the valve sheet therebetween, and in the state where the direction of a flow channel for discharging or storing contents with respect to the sealed bag is set to the up-down direction, the left end sides of the both outer sheets are heat-sealed to each other and the right end sides of the outer sheets are heat-sealed to each other, the base end side of the valve sheet is fixed to the inner surface of one outer sheet, and the portion between the inner surface of the other outer sheet and the surface on the opposite side of the fixed surface of the valve sheet forms a flow channel.
When this check valve is attached to the sealed bag, both inner surfaces near the opening of the sealed bag are layered on the outer surfaces of both outer sheets and heat-sealed.
During the heat-sealing for attaching the check valve to the sealed bag, the valve sheet sandwiched between the both outer sheets must be prevented from being heat-sealed to the outer sheets.
Therefore, the inventor of the present invention considered a measure in which a heat-resistant ink is printed on the surface on which the flow channel of the valve sheet is formed.
As a heat-resistant ink, an ink considered safe to the human body is used, however, if the contents are foods, due to the contact of the ink with the contents, safety cannot be guaranteed, and this is not permitted.
A manufacturer of the check valve must place an order with an outsourced printer for printing of the heat-resistant ink, and the number of lots to be ordered must be large to a certain extent, so it is difficult to place an order with an outsourced printer for printing of the heat-resistant ink in a small number of lots.
Therefore, further, instead of printing of a heat-resistant ink, the inventor of the present invention considered that the valve sheet was formed by laminating a film (sheet) made of a material that was not heat-weldable to the outer sheets on the surface constituting the flow channel of the valve sheet.
However, by providing a film that is not heat-weldable, problems have been found that, when manufacturing the valve, the inner surfaces of the outer sheets constituting the flow channel and the valve sheet opposed to the inner surfaces cannot be reliably heat-sealed, and non-sealed portions that are not sealed are formed between the left end sides of the outer sheets and the valve sheet and between the right end sides of the outer sheets and the valve sheet, and at these non-sealed portions, the airtightness of the sealed bag is lost, and contents leak or the outside air is introduced into the sealed bag from the non-sealed portions.